STI shifters move in two separate axes:
- the traditional 'toward you' / 'away from you' axis for braking; and
- a left / right axis for changing gear.

The STI unit allows movement in both axes at the same time so you can simultaneously change gear and brake. For instance you could at the same time:
- change from big ring to little ring at the front;
- rear brake;
- change down a couple of sprockets at the back; and
- front brake.

The STI shifter has a standard main lever that actuates the brakes and shifts in one direction. There is a secondary shift lever (kind of parallel to the main one) that changes in the opposite direction. The exact format depends on Tiagra, Ultegra, etc.

Don't sweat it, they are totally intuitive to use, it will take you all of three seconds to learn even if you are incredibly dull like me.

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Ha ha, you'd think that wouldn't you? I must be in the far left hand corner of the bell-shaped IQ curve 'cos when I first got my road bike I figured out how to change up a gear straightaway, but spent the following half an hour trying to pedal in top gear because I couldn't figure out how to change down!

Popped into the LBS to check out the bike on Saturday, and whilst I had rought idea of how they worked from all your descriptions, it was enlightening to actually flick the levers and see it in action.

I've got to say, I expected to have to put far more pressure on the levers to change gears; but then I take it Ultegra kit is pretty smooth and light in operation anyway? (haven't actually ridden it yet, just drooled over it!)